Here's why:

* Mid-ocean ridges are underwater mountain ranges where new oceanic crust is created.

* Magma from the Earth's mantle rises to the surface at these ridges, cools and solidifies, forming new crust.

* This new crust then pushes older crust away from the ridge, causing the seafloor to spread.

Think of it like a zipper opening: magma rises at the ridge (zipper), pushing the plates apart and creating new crust (the zipper teeth).